It is known that the blankets for offset printing comprise generally a support lined with one or several successive layers, themselves covered with an outer or printing layer commonly called a lithographic layer.
The lithographic layer is made from an incompressible material, such as nitrile rubber so as to obtain a good offset printing quality. However, such a lithographic layer made from an incompressible hard material exhibits a certain number of inconveniences. For example, it is not effective in absorbing irregularities of the subjacent layer which may result in printing defects.
Moreover, during the recto-verso printing of a sheet of paper passing between two printing cylinders or blankets, there occurs due to the incompressibility of the lithographic layer, a jumping up of the material of this layer which results in folds on either side of the contact zone between both blankets, i.e., at the ingress of the paper into the contact area and at the egress of the paper from this area. These folds are more marked as the lithographic layer is thicker as is the case for most of the lithographic layers used with the blankets of the prior art. Such folds produce, as is understandable, printing defects on the paper.
It should also be pointed out that with an incompressible lithographic layer (i.e., a layer deformable but nevertheless rigid in compression to preserve its volume), one is confronted with printing defects constituted by a slight shift of the colors on the paper, which imparts to the printed image fuzziness or separation of the colors. Once again, this is due to the deformation of the lithographic layer generating folds which somewhat deform the paper to be printed.
It should further be pointed out that during the manufacture of an incompressible lithographic layer, the finish of this layer, i.e., the polishing steps leaves, in spite of all the precautions taken, wavelets upon the external face of the layer, which of course may be result in defects during the printing of the paper.
The object of the present invention is to solve in particular the problems and inconveniences referred to herein above by providing the lithographic layer with some degree of compressibility. While incompressible lithographic layers of the prior art were reputed to possess maximum advantages during printing, the lithographic layer of the present invention, unlike any expectation, imparts to the layer very substantial if not exceptional advantages.